Move StringImplementation from coreimpl to core as _StringImpl.

runtime/lib/string_base.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
 class _StringBase {
 
   factory _StringBase._uninstantiable() {
     throw new UnsupportedError(
         "_StringBase can't be instaniated");
   }
 
   int get hashCode native "String_getHashCode";
 
   /**
    *  Create the most efficient string representation for specified
    *  [codePoints].
    */
   static String createFromCharCodes(List<int> charCodes) {
-    // TODO(ajohnsen): Add fast path once string_base is in core.
-    int len = charCodes.length;
-    var objectArray = new List(len);
-    for (int i = 0; i < len; i++) {
-      objectArray[i] = charCodes[i];
+    _ObjectArray objectArray;
+    if (charCodes is _ObjectArray) {
+      objectArray = charCodes;
+    } else {
+      int len = charCodes.length;
+      objectArray = new _ObjectArray(len);
+      for (int i = 0; i < len; i++) {
+        objectArray[i] = charCodes[i];
+      }
     }
     return _createFromCodePoints(objectArray);
   }
 
   static String _createFromCodePoints(List<int> codePoints)
       native "StringBase_createFromCodePoints";
 
   String operator [](int index) native "String_charAt";
 
   int charCodeAt(int index) native "String_charCodeAt";
@@ skipping 287 matching lines @@
       int j = 1;
       for (int i = 1; i < length; i++) {
         stringsList[j++] = separator;
         stringsList[j++] = strings[i];
       }
     }
     return concatAll(stringsList);
   }
 
   static String concatAll(List<String> strings) {
-    // TODO(ajohnsen): Add fast path once string_base is in core.
-    int len = strings.length;
-    var stringsArray = new List(len);
-    for (int i = 0; i < len; i++) {
-      stringsArray[i] = strings[i];
+    _ObjectArray stringsArray;
+    if (strings is _ObjectArray) {
+      stringsArray = strings;
+    } else {
+      int len = strings.length;
+      stringsArray = new _ObjectArray(len);
+      for (int i = 0; i < len; i++) {
+        stringsArray[i] = strings[i];
+      }
     }
     return _concatAll(stringsArray);
   }
 
   static String _concatAll(List<String> strings)
       native "Strings_concatAll";
 }
 
 
 class _OneByteString extends _StringBase implements String {
@@ skipping 120 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String str;
   final String pattern;
 }