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

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

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 313d2cb43c257ae90db0aa31a2a5eb0597f33a01..4cce9fd08b3259ce6a3717ab69e7bb19face72a9 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -60,6 +60,8 @@ import 'dart:_interceptors';
 import 'dart:_internal' as _symbol_dev;
 import 'dart:_internal' show MappedIterable;
 
+import 'dart:_native_typed_data';
+
 import 'dart:_js_names' show
     extractKeys,
     mangledNames,
@@ -764,19 +766,17 @@ class Primitives {
   // This is to avoid stack overflows due to very large argument arrays in
   // apply().  It fixes http://dartbug.com/6919
   static String _fromCharCodeApply(List<int> array) {
-    String result = "";
     const kMaxApply = 500;
     int end = array.length;
-    for (var i = 0; i < end; i += kMaxApply) {
-      var subarray;
-      if (end <= kMaxApply) {
-        subarray = array;
-      } else {
-        subarray = JS('JSExtendableArray', r'#.slice(#, #)', array,
-                      i, i + kMaxApply < end ? i + kMaxApply : end);
-      }
-      result = JS('String', '# + String.fromCharCode.apply(#, #)',
-                  result, null, subarray);
+    if (end <= kMaxApply) {
+      return JS('String', r'String.fromCharCode.apply(null, #)', array);
+    }
+    String result = '';
+    for (int i = 0; i < end; i += kMaxApply) {
+      int chunkEnd = (i + kMaxApply < end) ? i + kMaxApply : end;
+      result = JS('String',
+          r'# + String.fromCharCode.apply(null, #.slice(#, #))',
+          result, array, i, chunkEnd);
     }
     return result;
   }
@@ -806,6 +806,24 @@ class Primitives {
     return _fromCharCodeApply(charCodes);
   }
 
+  // [start] and [end] are validated.
+  static String stringFromNativeUint8List(
+      NativeUint8List charCodes, int start, int end) {
+    const kMaxApply = 500;
+    if (end <= kMaxApply && start == 0 && end == charCodes.length) {
+      return JS('String', r'String.fromCharCode.apply(null, #)', charCodes);
+    }
+    String result = '';
+    for (int i = start; i < end; i += kMaxApply) {
+      int chunkEnd = (i + kMaxApply < end) ? i + kMaxApply : end;
+      result = JS('String',
+          r'# + String.fromCharCode.apply(null, #.subarray(#, #))',
+          result, charCodes, i, chunkEnd);
+    }
+    return result;
+  }
+
+
   static String stringFromCharCode(charCode) {
     if (0 <= charCode) {
       if (charCode <= 0xffff) {