Let Uint16List.setRange detect a String.codeUnits object, and copy directly from the string.

runtime/lib/typed_data.dart

 // Copyright (c) 2013, 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.
 
 // patch classes for Int8List ..... Float64List and ByteData implementations.
 
 import "dart:_internal";
 import 'dart:math' show Random;
 
 patch class Int8List {
@@ skipping 811 matching lines @@
   void _setFloat32x4(int offsetInBytes, Float32x4 value)
       native "TypedData_SetFloat32x4";
 
   Int32x4 _getInt32x4(int offsetInBytes) native "TypedData_GetInt32x4";
   void _setInt32x4(int offsetInBytes, Int32x4 value)
       native "TypedData_SetInt32x4";
 
   Float64x2 _getFloat64x2(int offsetInBytes) native "TypedData_GetFloat64x2";
   void _setFloat64x2(int offsetInBytes, Float64x2 value)
       native "TypedData_SetFloat64x2";
+
+  /**
+   * Stores the [CodeUnits] as UTF-16 units into this TypedData at
+   * positions [start]..[end] (uint16 indices).
+   */
+  void _setCodeUnits(CodeUnits units,
+                     int byteStart, int length, int skipCount) {
+    assert(byteStart + length * Uint16List.BYTES_PER_ELEMENT <= lengthInBytes);
+    String string = CodeUnits.stringOf(units);
+    int sliceEnd = skipCount + length;
+    RangeError.checkValidRange(skipCount, sliceEnd,
+                               string.length,
+                               "skipCount", "skipCount + length");
+    for (int i = 0; i < length; i++) {
+      _setUint16(byteStart + i * Uint16List.BYTES_PER_ELEMENT,
+                 string.codeUnitAt(skipCount + i));
+    }
+  }
 }
 
 
 class _Int8Array extends _TypedList with _IntListMixin implements Int8List {
   // Factory constructors.
 
   factory _Int8Array(int length) {
     return _new(length);
   }
 
@@ skipping 130 matching lines @@
     return _getIndexedInt16(index);
   }
 
   void operator[]=(int index, int value) {
     if (index < 0 || index >= length) {
       throw _newRangeError(index, length);
     }
     _setIndexedInt16(index, _toInt16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) == CodeUnits.cid) {
+      end = RangeError.checkValidRange(start, end, this.length);
+      int length = end - start;
+      int byteStart = this.offsetInBytes + start * Int16List.BYTES_PER_ELEMENT;
+      _setCodeUnits(iterable, byteStart, length, skipCount);
+    } else {
+      super.setRange(start, end, iterable, skipCount);
+    }
+  }
 
   // Method(s) implementing TypedData interface.
 
   int get elementSizeInBytes {
     return Int16List.BYTES_PER_ELEMENT;
   }
 
 
   // Internal utility methods.
 
@@ skipping 30 matching lines @@
     return _getIndexedUint16(index);
   }
 
   void operator[]=(int index, int value) {
     if (index < 0 || index >= length) {
       throw _newRangeError(index, length);
     }
     _setIndexedUint16(index, _toUint16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) == CodeUnits.cid) {
+      end = RangeError.checkValidRange(start, end, this.length);
+      int length = end - start;
+      int byteStart = this.offsetInBytes + start * Uint16List.BYTES_PER_ELEMENT;
+      _setCodeUnits(iterable, byteStart, length, skipCount);
+    } else {
+      super.setRange(start, end, iterable, skipCount);
+    }
+  }
 
   // Method(s) implementing the TypedData interface.
 
   int get elementSizeInBytes {
     return Uint16List.BYTES_PER_ELEMENT;
   }
 
 
   // Internal utility methods.
 
@@ skipping 1404 matching lines @@
   // Method(s) implementing the TypedData interface.
 
   int get lengthInBytes {
     return length * elementSizeInBytes;
   }
 
   ByteBuffer get buffer {
     return _typedData.buffer;
   }
 
-  final TypedData _typedData;
+  final _TypedList _typedData;
   final int offsetInBytes;
   final int length;
 }
 
 
 class _Int8ArrayView extends _TypedListView with _IntListMixin implements Int8List {
   // Constructor.
   _Int8ArrayView(ByteBuffer buffer, [int _offsetInBytes = 0, int _length])
     : super(buffer, _offsetInBytes,
             _defaultIfNull(_length,
@@ skipping 159 matching lines @@
   }
 
   void operator[]=(int index, int value) {
     if (index < 0 || index >= length) {
       throw _newRangeError(index, length);
     }
     _typedData._setInt16(offsetInBytes + (index * Int16List.BYTES_PER_ELEMENT),
                          _toInt16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) == CodeUnits.cid) {
+      end = RangeError.checkValidRange(start, end, this.length);
+      int length = end - start;
+      int byteStart = this.offsetInBytes + start * Int16List.BYTES_PER_ELEMENT;
+      _typedData._setCodeUnits(iterable, byteStart, length, skipCount);
+    } else {
+      super.setRange(start, end, iterable, skipCount);
+    }
+  }
 
   // Method(s) implementing TypedData interface.
 
   int get elementSizeInBytes {
     return Int16List.BYTES_PER_ELEMENT;
   }
 
 
   // Internal utility methods.
 
@@ skipping 28 matching lines @@
   }
 
   void operator[]=(int index, int value) {
     if (index < 0 || index >= length) {
       throw _newRangeError(index, length);
     }
     _typedData._setUint16(offsetInBytes + (index * Uint16List.BYTES_PER_ELEMENT),
                           _toUint16(value));
   }
 
+  void setRange(int start, int end, Iterable iterable, [int skipCount = 0]) {
+    if (ClassID.getID(iterable) == CodeUnits.cid) {
+      end = RangeError.checkValidRange(start, end, this.length);
+      int length = end - start;
+      int byteStart = this.offsetInBytes + start * Uint16List.BYTES_PER_ELEMENT;
+      _typedData._setCodeUnits(iterable, byteStart, length, skipCount);
+    } else {
+      super.setRange(start, end, iterable, skipCount);
+    }
+  }
 
   // Method(s) implementing TypedData interface.
 
   int get elementSizeInBytes {
     return Uint16List.BYTES_PER_ELEMENT;
   }
 
-
   // Internal utility methods.
 
   Uint16List _createList(int length) {
     return new Uint16List(length);
   }
 }
 
 
 class _Int32ArrayView extends _TypedListView with _IntListMixin implements Int32List {
   // Constructor.
@@ skipping 784 matching lines @@
     return value;
   }
   return object;
 }
 
 
 _newRangeError(int index, int length) {
   String message = "$index must be in the range [0..$length)";
   return new RangeError(message);
 }