Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/typechecker.dart

Index: sdk/lib/_internal/compiler/implementation/typechecker.dart
diff --git a/sdk/lib/_internal/compiler/implementation/typechecker.dart b/sdk/lib/_internal/compiler/implementation/typechecker.dart
index 7cef548d1372a830edae7f73de93caa0b08e48b9..b1fb8cd434fc7613472b666e0482a5b58f8e88e3 100644
--- a/sdk/lib/_internal/compiler/implementation/typechecker.dart
+++ b/sdk/lib/_internal/compiler/implementation/typechecker.dart
@@ -250,6 +250,7 @@ class TypeCheckerVisitor extends Visitor<DartType> {
 
   Node lastSeenNode;
   DartType expectedReturnType;
+  AsyncMarker currentAsyncMarker = AsyncMarker.SYNC;
 
   final ClassElement currentClass;
 
@@ -604,10 +605,13 @@ class TypeCheckerVisitor extends Visitor<DartType> {
       returnType = functionType.returnType;
       type = functionType;
     }
-    DartType previous = expectedReturnType;
+    DartType previousReturnType = expectedReturnType;
     expectedReturnType = returnType;
+    AsyncMarker previousAsyncMarker = currentAsyncMarker;
+    currentAsyncMarker = element.asyncMarker;
     analyze(node.body);
-    expectedReturnType = previous;
+    expectedReturnType = previousReturnType;
+    currentAsyncMarker = previousAsyncMarker;
     return type;
   }
 
@@ -1589,6 +1593,36 @@ class TypeCheckerVisitor extends Visitor<DartType> {
     return const DynamicType();
   }
 
+  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;
+  }
+
+  DartType visitYield(Yield node) {
+    DartType resultType = analyze(node.expression);
+    if (!node.hasStar) {
+      if (currentAsyncMarker.isAsync) {
+        resultType =
+            compiler.streamClass.thisType.createInstantiation(
+                <DartType>[resultType]);
+      } else {
+        resultType =
+            compiler.iterableClass.thisType.createInstantiation(
+                 <DartType>[resultType]);
+      }
+    }
+    checkAssignable(node, resultType, expectedReturnType);
+    return const StatementType();
+  }
+
   DartType visitTypeAnnotation(TypeAnnotation node) {
     return elements.getType(node);
   }