Use the code point iterator when computing 16-bit string hash codes.

runtime/vm/object.h

 // 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.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 3724 matching lines @@
   // All strings share the same maximum element count to keep things
   // simple.  We choose a value that will prevent integer overflow for
   // 2 byte strings, since it is the worst case.
   static const intptr_t kSizeofRawString = sizeof(RawObject) + (2 * kWordSize);
   static const intptr_t kMaxElements = kSmiMax / kTwoByteChar;
 
   class CodePointIterator : public ValueObject {
    public:
     explicit CodePointIterator(const String& str)
         : str_(str),
+          ch_(0),
           index_(-1),
-          ch_(-1) {
+          end_(str.Length()) {
+    }
+
+    CodePointIterator(const String& str, intptr_t start, intptr_t length)
+        : str_(str),
+          ch_(0),
+          index_(start - 1),
+          end_(start + length) {
+      ASSERT(start >= 0);
+      ASSERT(end_ <= str.Length());
     }
 
     int32_t Current() {
       ASSERT(index_ >= 0);
-      ASSERT(index_ < str_.Length());
+      ASSERT(index_ < end_);
       return ch_;
     }
 
     bool Next();
 
    private:
     const String& str_;
+    int32_t ch_;
     intptr_t index_;
-    int32_t ch_;
+    intptr_t end_;
     DISALLOW_IMPLICIT_CONSTRUCTORS(CodePointIterator);
   };
 
   intptr_t Length() const { return Smi::Value(raw_ptr()->length_); }
   static intptr_t length_offset() { return OFFSET_OF(RawString, length_); }
 
   virtual intptr_t Hash() const;
   static intptr_t hash_offset() { return OFFSET_OF(RawString, hash_); }
   static intptr_t Hash(const String& str, intptr_t begin_index, intptr_t len);
   static intptr_t Hash(const uint8_t* characters, intptr_t len);
@@ skipping 2222 matching lines @@
     if (this->CharAt(i) != str.CharAt(begin_index + i)) {
       return false;
     }
   }
   return true;
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_