Make dart2js String.split match the VM version.

sdk/lib/_internal/compiler/js_lib/js_string.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.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 65 matching lines @@
     if (startIndex < 0 || startIndex > this.length) {
       throw new RangeError.range(startIndex, 0, this.length);
     }
     return stringReplaceFirstUnchecked(this, from, to, startIndex);
   }
 
   List<String> split(Pattern pattern) {
     checkNull(pattern);
     if (pattern is String) {
       return JS('JSExtendableArray', r'#.split(#)', this, pattern);
-    } else if (pattern is JSSyntaxRegExp) {
+    } else if (pattern is JSSyntaxRegExp && regExpCaptureCount(pattern) == 0) {
       var re = regExpGetNative(pattern);
       return JS('JSExtendableArray', r'#.split(#)', this, re);
     } else {
-      throw "String.split(Pattern) UNIMPLEMENTED";
+      return _defaultSplit(pattern);
     }
   }
 
+  List<String> _defaultSplit(Pattern pattern) {
+    List<String> result = <String>[];
+    // End of most recent match. That is, start of next part to add to result.
+    int start = 0;
+    // Length of most recent match.
+    // Set >0, so no match on the empty string causes the result to be [""].
+    int length = 1;
+    for (var match in pattern.allMatches(this)) {
+      int matchStart = match.start;
+      int matchEnd = match.end;
+      length = matchEnd - matchStart;
+      if (length == 0 && start == matchStart) {
+        // An empty match right after another match is ignored.
+        // This includes an empty match at the start of the string.
+        continue;
+      }
+      int end = matchStart;
+      result.add(this.substring(start, end));
+      start = matchEnd;
+    }
+    if (start < this.length || length > 0) {
+      // An empty match at the end of the string does not cause a "" at the end.
+      // A non-empty match ending at the end of the string does add a "".
+      result.add(this.substring(start));
+    }
+    return result;
+  }
+
   bool startsWith(Pattern pattern, [int index = 0]) {
     checkInt(index);
     if (index < 0 || index > this.length) {
       throw new RangeError.range(index, 0, this.length);
     }
     if (pattern is String) {
       String other = pattern;
       int otherLength = other.length;
       int endIndex = index + otherLength;
       if (endIndex > length) return false;
@@ skipping 330 matching lines @@
  */
 class _CodeUnits extends UnmodifiableListBase<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }