Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/builder.dart b/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
index 923b6e5b7dc3b37e76b866d4a3699a1fbde9381d..abd5e29594869b5486b149f52854b583ddcb859b 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
@@ -5058,6 +5058,17 @@ class SsaBuilder extends ResolvedVisitor {
     }
   }
 
+  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.');