dart2js Uri patch: avoid unnecessary code-unit to String conversions

sdk/lib/_internal/js_runtime/lib/core_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/core_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/core_patch.dart b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
index a9832709b21b2fe46243d8e73e18eb8874e2c62c..d55524211342d233eaaec77493a5b3ad8833a037 100644
--- a/sdk/lib/_internal/js_runtime/lib/core_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/core_patch.dart
@@ -558,12 +558,12 @@ class Uri {
           ((canonicalTable[byte >> 4] & (1 << (byte & 0x0f))) != 0)) {
         result.writeCharCode(byte);
       } else if (spaceToPlus && byte == _SPACE) {
-        result.writeCharCode(_PLUS);
+        result.write('+');
       } else {
         const String hexDigits = '0123456789ABCDEF';
-        result.writeCharCode(_PERCENT);
-        result.writeCharCode(hexDigits.codeUnitAt(byte >> 4));
-        result.writeCharCode(hexDigits.codeUnitAt(byte & 0x0f));
+        result.write('%');
+        result.write(hexDigits[(byte >> 4) & 0x0f]);
+        result.write(hexDigits[byte & 0x0f]);

[Lasse Reichstein Nielsen, 2015/11/05 21:48:26]

I guess it's OK, but it's a sign that dart2js' StringBuffer needs optimization
for code units.

[sra1, 2015/11/06 01:55:10]

It is really hard to get JavaScript performance with char codes anywhere near
using strings, which have been much more heavily optimized in most
implementations.

Our StringBuffer implementation is so simple that this function now compiles to
the following.
Can you find the StringBuffer?

        Uri__uriEncode: function(canonicalTable, text, encoding, spaceToPlus) {
          var bytes, t1, i, t2, $byte, t3;
          if (encoding === C.Utf8Codec_false &&
$.$get$Uri__needsNoEncoding()._nativeRegExp.test(H.checkString(text)))
            return text;
          bytes = encoding.get$encoder().convert$1(text);
          for (t1 = bytes.length, i = 0, t2 = ""; i < t1; ++i) {
            $byte = bytes[i];
            if ($byte < 128) {
              t3 = $byte >>> 4;
              if (t3 >= 8)
                return H.ioore(canonicalTable, t3);
              t3 = (canonicalTable[t3] & C.JSInt_methods._shlPositive$1(1, $byte
& 15)) !== 0;
            } else
              t3 = false;
            if (t3)
              t2 += H.Primitives_stringFromCharCode($byte);
            else
              t2 = spaceToPlus && $byte === 32 ? t2 + "+" : t2 + "%" +
"0123456789ABCDEF"[$byte >>> 4 & 15] + "0123456789ABCDEF"[$byte & 15];
          }
          return t2.charCodeAt(0) == 0 ? t2 : t2;
        }

       }
     }
     return result.toString();