Recognize and special case String.fromCharCodes for Uint8List in JavaScript.

sdk/lib/_internal/compiler/js_lib/core_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/core_patch.dart b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
index 29ee7787287406b79905d8f436547aa66d6ec302..3fe172b646c748ea6c5f2a2bfc0ff4fefa7fd81a 100644
--- a/sdk/lib/_internal/compiler/js_lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/core_patch.dart
@@ -14,6 +14,8 @@ import 'dart:_js_helper' show patch,
                               stringJoinUnchecked,
                               objectHashCode;
 
+import 'dart:_native_typed_data' show NativeUint8List;
+
 String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 
 _symbolMapToStringMap(Map<Symbol, dynamic> map) {
@@ -275,26 +277,14 @@ class String {
   @patch
   factory String.fromCharCodes(Iterable<int> charCodes,
                                [int start = 0, int end]) {
-    // If possible, recognize typed lists too.
-    if (charCodes is! JSArray) {
-      return _stringFromIterable(charCodes, start, end);
-    }
 
-    List list = charCodes;
-    int len = list.length;
-    if (start < 0 || start > len) {
-      throw new RangeError.range(start, 0, len);
+    if (charCodes is JSArray) {
+      return _stringFromJSArray(charCodes, start, end);
     }
-    if (end == null) {
-      end = len;
-    } else if (end < start || end > len) {
-      throw new RangeError.range(end, start, len);
+    if (charCodes is NativeUint8List) {
+      return _stringFromUint8List(charCodes, start, end);
     }
-
-    if (start > 0 || end < len) {
-      list = list.sublist(start, end);
-    }
-    return Primitives.stringFromCharCodes(list);
+    return _stringFromIterable(charCodes, start, end);
   }
 
   @patch
@@ -308,6 +298,34 @@ class String {
         'String.fromEnvironment can only be used as a const constructor');
   }
 
+  static String _stringFromJSArray(List list, int start, int endOrNull) {
+    int len = list.length;
+    int end = _checkBounds(len, start, endOrNull);
+    if (start > 0 || end < len) {
+      list = list.sublist(start, end);
+    }
+    return Primitives.stringFromCharCodes(list);
+  }
+
+  static String _stringFromUint8List(
+      NativeUint8List charCodes, int start, int endOrNull) {
+    int len = charCodes.length;
+    int end = _checkBounds(len, start, endOrNull);
+    return Primitives.stringFromNativeUint8List(charCodes, start, end);
+  }
+
+  static int _checkBounds(int len, int start, int end) {
+    if (start < 0 || start > len) {
+      throw new RangeError.range(start, 0, len);
+    }
+    if (end == null) {
+      end = len;
+    } else if (end < start || end > len) {
+      throw new RangeError.range(end, start, len);
+    }
+    return end;
+  }
+
   static String _stringFromIterable(Iterable<int> charCodes,
                                     int start, int end) {
     if (start < 0) throw new RangeError.range(start, 0, charCodes.length);