Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/ssa/builder.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 ssa;
 
 /// A synthetic local variable only used with the SSA graph.
 ///
 /// For instance used for holding return value of function or the exception of a
 /// try-catch statement.
@@ skipping 5040 matching lines @@
 
   visitThrow(ast.Throw node) {
     visitThrowExpression(node.expression);
     if (isReachable) {
       handleInTryStatement();
       push(new HThrowExpression(pop()));
       isReachable = false;
     }
   }
 
+  visitYield(ast.Yield node) {
+    // Dummy implementation.
+    visit(node.expression);
+    pop();
+  }
+
+  visitAwait(ast.Await node) {
+    // Dummy implementation.
+    visit(node.expression);
+  }
+
   visitTypeAnnotation(ast.TypeAnnotation node) {
     compiler.internalError(node,
         'Visiting type annotation in SSA builder.');
   }
 
   visitVariableDefinitions(ast.VariableDefinitions node) {
     assert(isReachable);
     for (Link<ast.Node> link = node.definitions.nodes;
          !link.isEmpty;
          link = link.tail) {
@@ skipping 1487 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }