Refactoring of string literals. Implement static string addition.

frog/tests/leg/src/BuiltinInterceptorTest.dart

Index: frog/tests/leg/src/BuiltinInterceptorTest.dart
diff --git a/frog/tests/leg/src/BuiltinInterceptorTest.dart b/frog/tests/leg/src/BuiltinInterceptorTest.dart
index a5d617cf35a137cdcda32158f2d812f81b604566..0c69ead0f6441655778362b0b99d617eda4841d7 100644
--- a/frog/tests/leg/src/BuiltinInterceptorTest.dart
+++ b/frog/tests/leg/src/BuiltinInterceptorTest.dart
@@ -26,11 +26,9 @@ main() {
   String generated = compile(TEST_ONE, 'foo');
   Expect.isTrue(generated.contains("return a.length;"));
 
-  // TODO(lrn): Switch these two to isTrue when we support constant folding
-  // of string length.
   generated = compile(TEST_TWO, 'foo');
-  Expect.isFalse(generated.contains("return 3;"));
+  Expect.isTrue(generated.contains("return 3;"));
 
   generated = compile(TEST_THREE, 'foo');
-  Expect.isFalse(generated.contains("return 3;"));
+  Expect.isTrue(generated.contains("return 3;"));
 }