Improve performance of String split (use unchecked substring).

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 {
 
@@ skipping 35 matching lines @@
     return this.length == 0;
   }
 
   String concat(String other) native "String_concat";
 
   String toString() {
     return this;
   }
 
   bool operator ==(Object other) {
-    if (this === other) {
+    if (identical(this, other)) {
       return true;
     }
     if ((other is !String) ||
         (this.length != other.length)) {
       // TODO(5413632): Compare hash codes when both are present.
       return false;
     }
     return this.compareTo(other) == 0;
   }
 
@@ skipping 78 matching lines @@
 
     if ((startIndex < 0) || (startIndex > this.length)) {
       throw new RangeError.value(startIndex);
     }
     if ((endIndex < 0) || (endIndex > this.length)) {
       throw new RangeError.value(endIndex);
     }
     if (startIndex > endIndex) {
       throw new RangeError.value(startIndex);
     }
+    return _substringUnchecked(startIndex, endIndex);
+  }
+
+  String _substringUnchecked(int startIndex, int endIndex) {
+    assert(endIndex != null);
+    assert((startIndex >= 0) && (startIndex <= this.length));
+    assert((endIndex >= 0) && (endIndex <= this.length));
+    assert(startIndex <= endIndex);
+
     if (startIndex == endIndex) {
       return "";
     }
     if ((startIndex + 1) == endIndex) {
       return this[startIndex];
     }
-    return _substringUnchecked(startIndex, endIndex);
+    return _substringUncheckedNative(startIndex, endIndex);
   }
 
-  String _substringUnchecked(int startIndex, int endIndex)
+  String _substringUncheckedNative(int startIndex, int endIndex)
       native "StringBase_substringUnchecked";
 
   String trim() {
     final int len = this.length;
     int first = 0;
     for (; first < len; first++) {
       if (!_isWhitespace(this.charCodeAt(first))) {
         break;
       }
     }
@@ skipping 101 matching lines @@
     Iterator iterator = pattern.allMatches(this).iterator();
     if (length == 0 && iterator.hasNext) {
       // A matched empty string input returns the empty list.
       return <String>[];
     }
     List<String> result = new List<String>();
     int startIndex = 0;
     int previousIndex = 0;
     while (true) {
       if (startIndex == length || !iterator.hasNext) {
-        result.add(this.substring(previousIndex, length));
+        result.add(this._substringUnchecked(previousIndex, length));
         break;
       }
       Match match = iterator.next();
       if (match.start == length) {
-        result.add(this.substring(previousIndex, length));
+        result.add(this._substringUnchecked(previousIndex, length));
         break;
       }
       int endIndex = match.end;
       if (startIndex == endIndex && endIndex == previousIndex) {
         ++startIndex;  // empty match, advance and restart
         continue;
       }
-      result.add(this.substring(previousIndex, match.start));
+      result.add(this._substringUnchecked(previousIndex, match.start));
       startIndex = previousIndex = endIndex;
     }
     return result;
   }
 
   List<String> splitChars() {
     int len = this.length;
     final result = new List<String>(len);
     for (int i = 0; i < len; i++) {
       result[i] = this[i];
@@ skipping 61 matching lines @@
 
   // Checks for one-byte whitespaces only.
   // TODO(srdjan): Investigate if 0x85 (NEL) and 0xA0 (NBSP) are valid
   // whitespaces for one byte strings.
   bool _isWhitespace(int codePoint) {
     return
       (codePoint == 32) || // Space.
       ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
   }
 
-  String _substringUnchecked(int startIndex, int endIndex)
+  String _substringUncheckedNative(int startIndex, int endIndex)
       native "OneByteString_substringUnchecked";
 }
 
 
 class _TwoByteString extends _StringBase implements String {
   factory _TwoByteString._uninstantiable() {
     throw new UnsupportedError(
         "_TwoByteString can only be allocated by the VM");
   }
 
@@ skipping 97 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String str;
   final String pattern;
 }