Rename String.concat to operator+.

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

Index: sdk/lib/_internal/compiler/implementation/lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
index c271e1606fea07b7d644072a6cc41ca19440167e..d917230d923e3ca0f7a9f14f3b4df6700d6caa4d 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
@@ -25,11 +25,13 @@ class JSString implements String {
     return allMatchesInStringUnchecked(this, str);
   }
 
-  String concat(String other) {
+  String operator +(String other) {
     if (other is !String) throw new ArgumentError(other);
     return JS('String', r'# + #', this, other);
   }
 
+  String concat(String other) => this + other;
+
   bool endsWith(String other) {
     checkString(other);
     int otherLength = other.length;