Made use of extension Add methods in Expression lambdas a compiler error.

Expression Trees specifically require that element initializer methods are instance methods.
Fixes #310

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -3922,6 +3922,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to An expression tree lambda may not contain an extension collection element initializer..
+        /// </summary>
+        internal static string ERR_ExtensionCollectionElementInitializerInExpressionTree {
+            get {
+                return ResourceManager.GetString("ERR_ExtensionCollectionElementInitializerInExpressionTree", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Extension methods must be defined in a top level static class; {0} is a nested class.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4544,6 +4544,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_DictionaryInitializerInExpressionTree" xml:space="preserve">
     <value>An expression tree lambda may not contain a dictionary initializer.</value>
   </data>
+  <data name="ERR_ExtensionCollectionElementInitializerInExpressionTree" xml:space="preserve">
+    <value>An expression tree lambda may not contain an extension collection element initializer.</value>
+  </data>
   <data name="IDS_FeatureNameof" xml:space="preserve">
     <value>nameof operator</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1289,7 +1289,7 @@ internal enum ErrorCode
         ERR_NullPropagatingOpInExpressionTree = 8072,
         WRN_NubExprIsConstBool2 = 8073,
         ERR_DictionaryInitializerInExpressionTree = 8074,
-        // available: 8075,
+        ERR_ExtensionCollectionElementInitializerInExpressionTree = 8075,
         ERR_UnclosedExpressionHole = 8076,
         ERR_SingleLineCommentInExpressionHole = 8077,
         ERR_InsufficientStack = 8078,

src/Compilers/CSharp/Portable/Lowering/DiagnosticsPass_ExpressionTrees.cs

@@ -289,6 +289,11 @@ public override BoundNode VisitCall(BoundCall node)
 
         public override BoundNode VisitCollectionElementInitializer(BoundCollectionElementInitializer node)
         {
+            if (_inExpressionLambda && node.AddMethod.IsStatic)
+            {
+                Error(ErrorCode.ERR_ExtensionCollectionElementInitializerInExpressionTree, node);
+            }
+
             VisitCall(node.AddMethod, null, node.Arguments, default(ImmutableArray<RefKind>), default(ImmutableArray<string>), node.Expanded, node);
             return base.VisitCollectionElementInitializer(node);
         }

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_ObjectOrCollectionInitializerExpression.cs

@@ -163,10 +163,8 @@ private BoundExpression MakeCollectionInitializer(BoundExpression rewrittenRecei
                 // the add method was found as an extension method.  Replace the implicit receiver (first argument) with the rewritten receiver.
                 Debug.Assert(addMethod.IsStatic && addMethod.IsExtensionMethod);
                 Debug.Assert(rewrittenArguments[0].Kind == BoundKind.ImplicitReceiver);
-                var newArgs = ArrayBuilder<BoundExpression>.GetInstance();
-                newArgs.AddRange(rewrittenArguments);
-                newArgs[0] = rewrittenReceiver;
-                rewrittenArguments = newArgs.ToImmutableAndFree();
+                Debug.Assert(!_inExpressionLambda, "Expression trees do not support extension Add");
+                rewrittenArguments = rewrittenArguments.SetItem(0, rewrittenReceiver);
                 rewrittenReceiver = null;
             }
 

src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs

@@ -22237,6 +22237,46 @@ public void DictionaryInitializerInExprLambda1()
         {
             var text = @"
 using System;
+using System.Collections.Generic;
+using System.Linq.Expressions;
+
+namespace ConsoleApplication31
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            var o = new Foo();
+            var x = o.E.Compile()().Pop();
+            System.Console.WriteLine(x);
+        }
+    }
+
+    static class StackExtensions
+    {
+        public static void Add<T>(this Stack<T> s, T x) => s.Push(x);
+    }
+
+    class Foo
+    {
+        public Expression<Func<Stack<int>>> E = () => new Stack<int> { 42 };
+    }
+}
+
+";
+            CreateCompilationWithMscorlib45(text, new[] { SystemRef_v4_0_30319_17929, SystemCoreRef_v4_0_30319_17929, CSharpRef }).VerifyDiagnostics(
+    // (25,72): error CS8075: An expression tree lambda may not contain an extension collection element initializer.
+    //         public Expression<Func<Stack<int>>> E = () => new Stack<int> { 42 };
+    Diagnostic(ErrorCode.ERR_ExtensionCollectionElementInitializerInExpressionTree, "42").WithLocation(25, 72)
+               );
+        }
+
+        [WorkItem(310, "https://github.com/dotnet/roslyn/issues/310")]
+        [Fact]
+        public void ExtensionElementInitializerInExpressionLambda()
+        {
+            var text = @"
+using System;
 using System.Collections;
 using System.Linq.Expressions;
 class C