Revert "Add StringSink and update StringBuffer."

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

Index: sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index 75724a76604ae1b01268e27002c990408e917ec9..844ee78c9aeec4f7d3beff36806de95ad2b3fdcf 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -347,10 +347,6 @@ class Primitives {
     return _fromCharCodeApply(charCodes);
   }
 
-  static String stringConcatUnchecked(String string1, String string2) {
-    return JS('String', r'# + #', string1, string2);
-  }
-
   static String getTimeZoneName(receiver) {
     // When calling toString on a Date it will emit the timezone in parenthesis.
     // Example: "Wed May 16 2012 21:13:00 GMT+0200 (CEST)".