dart2js: Support new const operations on strings.

sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart b/sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart
index 9ed094be68718094acf5ecab3261efe666ab9320..b13a4b73b649fadf21944bf7e9d630a861e00b98 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart
@@ -96,6 +96,21 @@ class JavaScriptNegateOperation implements UnaryOperation {
   }
 }
 
+class JavaScriptAddOperation implements BinaryOperation {
+  final _addOperation = const AddOperation();
+  String get name => _addOperation.name;
+
+  const JavaScriptAddOperation();
+
+  Constant fold(Constant left, Constant right) {
+    Constant result = _addOperation.fold(left, right);
+    if (result == null || result.isString) return result;

[Johnni Winther, 2014/09/24 09:29:22]

This is not needed since convertToJavaScriptConstant is the identity function on
strings.

[floitsch, 2014/09/24 12:53:19]

True, but it is typed to take a NumConstant and return a NumConstant. I could
either change that, or have the test here.
What do you prefer?

[Johnni Winther, 2014/09/25 09:07:56]

Then I think I'd prefer to only call convertToJavaScriptConstant `if (result !=
null && result.isNum) ...`

[floitsch, 2014/09/25 12:29:05]

Done.

+    return JAVA_SCRIPT_CONSTANT_SYSTEM.convertToJavaScriptConstant(result);
+  }
+
+  apply(left, right) => _addOperation.apply(left, right);
+}
+
 class JavaScriptBinaryArithmeticOperation implements BinaryOperation {
   final BinaryOperation dartArithmeticOperation;
 
@@ -145,7 +160,7 @@ class JavaScriptConstantSystem extends ConstantSystem {
   final int BITS31 = 0x8FFFFFFF;
   final int BITS32 = 0xFFFFFFFF;
 
-  final add = const JavaScriptBinaryArithmeticOperation(const AddOperation());
+  final add = const JavaScriptAddOperation();
   final bitAnd = const JavaScriptBinaryBitOperation(const BitAndOperation());
   final bitNot = const JavaScriptBitNotOperation();
   final bitOr = const JavaScriptBinaryBitOperation(const BitOrOperation());