Move StringImplementation from coreimpl to core as _StringImpl.

lib/compiler/implementation/lib/string_helper.dart

Index: lib/compiler/implementation/lib/string_helper.dart
diff --git a/lib/compiler/implementation/lib/string_helper.dart b/lib/compiler/implementation/lib/string_helper.dart
index eaf132c1b25710435aa79aa800baa06be5f64fa1..f84fdeef55f438af7eca1bf7ed060deb09db2982 100644
--- a/lib/compiler/implementation/lib/string_helper.dart
+++ b/lib/compiler/implementation/lib/string_helper.dart
@@ -105,7 +105,7 @@ stringReplaceAllUnchecked(receiver, from, to) {
   } else {
     checkNull(from);
     // TODO(floitsch): implement generic String.replace (with patterns).
-    throw "StringImplementation.replaceAll(Pattern) UNIMPLEMENTED";
+    throw "String.replaceAll(Pattern) UNIMPLEMENTED";
   }
 }
 
@@ -118,7 +118,7 @@ stringReplaceFirstUnchecked(receiver, from, to) {
   } else {
     checkNull(from);
     // TODO(floitsch): implement generic String.replace (with patterns).
-    throw "StringImplementation.replace(Pattern) UNIMPLEMENTED";
+    throw "String.replace(Pattern) UNIMPLEMENTED";
   }
 }
 
@@ -129,7 +129,7 @@ stringSplitUnchecked(receiver, pattern) {
     var re = regExpGetNative(pattern);
     return JS('List', r'#.split(#)', receiver, re);
   } else {
-    throw "StringImplementation.split(Pattern) UNIMPLEMENTED";
+    throw "String.split(Pattern) UNIMPLEMENTED";
   }
 }