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 f5f27038c3a89b802b190844d48c290e52699242..1b8fef0c3820b80b461b3990df7112552fd31cf5 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, JSIndexable {
     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;