Fix await parsing.

pkg/compiler/lib/src/typechecker.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name => "Type checker";
 
@@ skipping 1583 matching lines @@
     }
     return const StatementType();
   }
 
   DartType visitThrow(Throw node) {
     // TODO(johnniwinther): Handle reachability.
     analyze(node.expression);
     return const DynamicType();
   }
 
+  /// Flatten [type] by recursively removing enclosing `Future` annotations.
+  ///
+  /// For instance:
+  ///     flatten(T) = T
+  ///     flatten(Future<T>) = T
+  ///     flatten(Future<Future<T>>) = T
+  ///
+  /// This method is used in the static typing of await and type checking of
+  /// return.
+  DartType flatten(DartType type) {
+    if (type is InterfaceType) {
+      InterfaceType futureType = type.asInstanceOf(compiler.futureClass);
+      if (futureType != null) {
+        return flatten(futureType.typeArguments.first);
+      }
+    }
+    return type;
+  }
+
   DartType visitAwait(Await node) {
     DartType expressionType = analyze(node.expression);
-    DartType resultType = expressionType;
-    if (expressionType is InterfaceType) {
-      InterfaceType futureType =
-          expressionType.asInstanceOf(compiler.futureClass);
-      if (futureType != null) {
-        resultType = futureType.typeArguments.first;
-      }
-    }
-    return resultType;
+    return flatten(expressionType);
   }
 
   DartType visitYield(Yield node) {
     DartType resultType = analyze(node.expression);
     if (!node.hasStar) {
       if (currentAsyncMarker.isAsync) {
         resultType =
             compiler.streamClass.thisType.createInstantiation(
                 <DartType>[resultType]);
       } else {
@@ skipping 203 matching lines @@
 
   visitTypedef(Typedef node) {
     // Do not typecheck [Typedef] nodes.
   }
 
   visitNode(Node node) {
     compiler.internalError(node,
         'Unexpected node ${node.getObjectDescription()} in the type checker.');
   }
 }