Added a default entrypoint variable to match android syntax. (#12370)

Made creating and using a FlutterEngine a bit easier, to try to get it get it as easy to use as Android's equivalent.
* Added a default entrypoint variable.
* I added `run` to and `initWithName:` to FlutterEngine.

shell/platform/darwin/ios/framework/Headers/FlutterEngine.h

@@ -18,6 +18,12 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+/**
+ * The dart entrypoint that is associated with `main()`.  This is to be used as an argument to the
+ * `runWithEntrypoint*` methods.
+ */
+extern NSString* const FlutterDefaultDartEntrypoint;
+
 /**
  * The FlutterEngine class coordinates a single instance of execution for a
  * `FlutterDartProject`.  It may have zero or one `FlutterViewController` at a
@@ -41,6 +47,26 @@ NS_ASSUME_NONNULL_BEGIN
  */
 FLUTTER_EXPORT
 @interface FlutterEngine : NSObject <FlutterTextureRegistry, FlutterPluginRegistry>
+
+/**
+ * Initialize this FlutterEngine.
+ *
+ * The engine will execute the project located in the bundle with the identifier
+ * "io.flutter.flutter.app" (the default for Flutter projects).
+ *
+ * A newly initialized engine will not run until either `-runWithEntrypoint:` or
+ * `-runWithEntrypoint:libraryURI:` is called.
+ *
+ * FlutterEngine created with this method will have allowHeadlessExecution set to `YES`.
+ * This means that the engine will continue to run regardless of whether a `FlutterViewController`
+ * is attached to it or not, until `-destroyContext:` is called or the process finishes.
+ *
+ * @param labelPrefix The label prefix used to identify threads for this instance. Should
+ *   be unique across FlutterEngine instances, and is used in instrumentation to label
+ *   the threads used by this FlutterEngine.
+ */
+- (instancetype)initWithName:(NSString*)labelPrefix;
+
 /**
  * Initialize this FlutterEngine with a `FlutterDartProject`.
  *
@@ -91,6 +117,17 @@ FLUTTER_EXPORT
 
 + (instancetype)new NS_UNAVAILABLE;
 
+/**
+ * Runs a Dart program on an Isolate from the main Dart library (i.e. the library that
+ * contains `main()`), using `main()` as the entrypoint (the default for Flutter projects).
+ *
+ * The first call to this method will create a new Isolate. Subsequent calls will return
+ * immediately.
+ *
+ * @return YES if the call succeeds in creating and running a Flutter Engine instance; NO otherwise.
+ */
+- (BOOL)run;
+
 /**
  * Runs a Dart program on an Isolate from the main Dart library (i.e. the library that
  * contains `main()`).
@@ -99,10 +136,10 @@ FLUTTER_EXPORT
  * immediately.
  *
  * @param entrypoint The name of a top-level function from the same Dart
- *   library that contains the app's main() function.  If this is nil, it will
- *   default to `main()`.  If it is not the app's main() function, that function
- *   must be decorated with `@pragma(vm:entry-point)` to ensure the method is not
- *   tree-shaken by the Dart compiler.
+ *   library that contains the app's main() function.  If this is FlutterDefaultDartEntrypoint (or
+ *   nil) it will default to `main()`.  If it is not the app's main() function, that function must
+ *   be decorated with `@pragma(vm:entry-point)` to ensure the method is not tree-shaken by the Dart
+ *   compiler.
  * @return YES if the call succeeds in creating and running a Flutter Engine instance; NO otherwise.
  */
 - (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint;
@@ -114,10 +151,10 @@ FLUTTER_EXPORT
  * The first call to this method will create a new Isolate. Subsequent calls will return
  * immediately.
  *
- * @param entrypoint The name of a top-level function from a Dart library.  If nil, this will
- *   default to `main()`.  If it is not the app's main() function, that function
- *   must be decorated with `@pragma(vm:entry-point)` to ensure the method is not
- *   tree-shaken by the Dart compiler.
+ * @param entrypoint The name of a top-level function from a Dart library.  If this is
+ *   FlutterDefaultDartEntrypoint (or nil); this will default to `main()`.  If it is not the app's
+ *   main() function, that function must be decorated with `@pragma(vm:entry-point)` to ensure the
+ *   method is not tree-shaken by the Dart compiler.
  * @param uri The URI of the Dart library which contains the entrypoint method.  IF nil,
  *   this will default to the same library as the `main()` function in the Dart program.
  * @return YES if the call succeeds in creating and running a Flutter Engine instance; NO otherwise.

shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -27,6 +27,8 @@
 #import "flutter/shell/platform/darwin/ios/ios_surface.h"
 #import "flutter/shell/platform/darwin/ios/platform_view_ios.h"
 
+NSString* const FlutterDefaultDartEntrypoint = nil;
+
 @interface FlutterEngine () <FlutterTextInputDelegate, FlutterBinaryMessenger>
 // Maintains a dictionary of plugin names that have registered with the engine.  Used by
 // FlutterEngineRegistrar to implement a FlutterPluginRegistrar.
@@ -70,6 +72,10 @@
   FlutterBinaryMessengerRelay* _binaryMessenger;
 }
 
+- (instancetype)initWithName:(NSString*)labelPrefix {
+  return [self initWithName:labelPrefix project:nil allowHeadlessExecution:YES];
+}
+
 - (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)project {
   return [self initWithName:labelPrefix project:project allowHeadlessExecution:YES];
 }
@@ -429,6 +435,10 @@
   return _shell != nullptr;
 }
 
+- (BOOL)run {
+  return [self runWithEntrypoint:FlutterDefaultDartEntrypoint libraryURI:nil];
+}
+
 - (BOOL)runWithEntrypoint:(NSString*)entrypoint libraryURI:(NSString*)libraryURI {
   if ([self createShell:entrypoint libraryURI:libraryURI]) {
     [self launchEngine:entrypoint libraryURI:libraryURI];