Trivial fixes to dart:ui docs. (#3689)

lib/ui/semantics.dart

@@ -186,7 +186,7 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
   /// system.
   ///
   /// The `transform` is a matrix that maps this node's coodinate system into
-  /// its parent's coordate system.
+  /// its parent's coordinate system.
   void updateNode({
     int id,
     int flags,
@@ -197,7 +197,7 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
     Int32List children
   }) {
     if (transform.length != 16)
-      throw new ArgumentError("[transform] must have 16 entries.");
+      throw new ArgumentError('transform argument must have 16 entries.');
     _updateNode(id,
                 flags,
                 actions,

lib/ui/window.dart

@@ -110,6 +110,16 @@ class Window {
   /// The number of device pixels for each logical pixel. This number might not
   /// be a power of two. Indeed, it might not even be an integer. For example,
   /// the Nexus 6 has a device pixel ratio of 3.5.
+  ///
+  /// Device pixels are also referred to as physical pixels. Logical pixels are
+  /// also referred to as device-independent or resolution-independent pixels.
+  ///
+  /// By definition, there are roughly 38 logical pixels per centimeter, or
+  /// about 96 logical pixels per inch, of the physical display. The value
+  /// returned by [devicePixelRatio] is ultimately obtained either from the
+  /// hardware itself, the device drivers, or a hard-coded value stored in the
+  /// operating system or firmware, and may be inaccurate, sometimes by a
+  /// significant margin.
   double get devicePixelRatio => _devicePixelRatio;
   double _devicePixelRatio = 1.0;
 
@@ -127,7 +137,9 @@ class Window {
   WindowPadding _padding = WindowPadding.zero;
 
   /// A callback that is invoked whenever the [devicePixelRatio],
-  /// [physicalSize], or [padding] values change.
+  /// [physicalSize], or [padding] values change, for example when the device is
+  /// rotated or when the application is resized (e.g. when showing applications
+  /// side-by-side on Android).
   VoidCallback onMetricsChanged;
 
   /// The system-reported locale.