dart2js: Support new const operations on strings.

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

Index: sdk/lib/_internal/compiler/implementation/compile_time_constants.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compile_time_constants.dart b/sdk/lib/_internal/compiler/implementation/compile_time_constants.dart
index e717c15d48684de33dc8b8c17e651c1046ac53b9..776e1a9b22e51dd7c30555b619acf9ee2a931892 100644
--- a/sdk/lib/_internal/compiler/implementation/compile_time_constants.dart
+++ b/sdk/lib/_internal/compiler/implementation/compile_time_constants.dart
@@ -409,6 +409,13 @@ class CompileTimeConstantEvaluator extends Visitor {
         assert(elements.isTypeLiteral(send));
         return makeTypeConstant(elements.getTypeLiteralType(send));
       } else if (send.receiver != null) {
+        if (send.selector.asIdentifier().source == "length") {
+          Constant left = evaluate(send.receiver);
+          if (left != null && left.isString) {
+            StringConstant string = left;
+            return constantSystem.createInt(string.value.length);
+          }
+        }
         // Fall through to error handling.
       } else if (!Elements.isUnresolved(element)
                  && element.isVariable
@@ -418,7 +425,7 @@ class CompileTimeConstantEvaluator extends Visitor {
       }
       return signalNotCompileTimeConstant(send);
     } else if (send.isCall) {
-      if (identical(element, compiler.identicalFunction)
+      if (element == compiler.identicalFunction
           && send.argumentCount() == 2) {
         Constant left = evaluate(send.argumentsNode.nodes.head);
         Constant right = evaluate(send.argumentsNode.nodes.tail.head);