Don't copy text in _uriEncode if not necessary.

sdk/lib/core/uri.dart

 // 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.
 
 part of dart.core;
 
 /**
  * A parsed URI, such as a URL.
  *
  * **See also:**
@@ skipping 2242 matching lines @@
 
   /**
    * This is the internal implementation of JavaScript's encodeURI function.
    * It encodes all characters in the string [text] except for those
    * that appear in [canonicalTable], and returns the escaped string.
    */
   static String _uriEncode(List<int> canonicalTable,
                            String text,
                            {Encoding encoding: UTF8,
                             bool spaceToPlus: false}) {
-    byteToHex(byte, buffer) {
-      const String hex = '0123456789ABCDEF';
-      buffer.writeCharCode(hex.codeUnitAt(byte >> 4));
-      buffer.writeCharCode(hex.codeUnitAt(byte & 0x0f));
-    }
-
-    // Encode the string into bytes then generate an ASCII only string
-    // by percent encoding selected bytes.
-    StringBuffer result = new StringBuffer();
     var bytes = encoding.encode(text);
+    StringBuffer result;
+    int start = 0;
     for (int i = 0; i < bytes.length; i++) {
       int byte = bytes[i];
       if (byte < 128 &&
           ((canonicalTable[byte >> 4] & (1 << (byte & 0x0f))) != 0)) {
-        result.writeCharCode(byte);
-      } else if (spaceToPlus && byte == _SPACE) {
+        continue;
+      }
+      result ??= new StringBuffer();
+      for (int j = start; j < i; j++) {
+        result.writeCharCode(bytes[j]);
+      }
+      if (spaceToPlus && byte == _SPACE) {
         result.writeCharCode(_PLUS);
       } else {
         result.writeCharCode(_PERCENT);
-        byteToHex(byte, result);
+        const String hex = '0123456789ABCDEF';
+        result.writeCharCode(hex.codeUnitAt(byte >> 4));
+        result.writeCharCode(hex.codeUnitAt(byte & 0x0f));
+      }
+      start = i + 1;
+    }
+    if (result == null) {
+      if (identical(encoding, ASCII) ||
+          identical(encoding, LATIN1) ||
+          identical(encoding, UTF8)) {
+        return text;
+      }
+      return new String.fromCharCodes(bytes);
+    }
+    if (start < bytes.length) {
+      for (int j = start; j < bytes.length; j++) {
+        result.writeCharCode(bytes[j]);
       }
     }
     return result.toString();
   }
 
   /**
    * Convert a byte (2 character hex sequence) in string [s] starting
    * at position [pos] to its ordinal value
    */
   static int _hexCharPairToByte(String s, int pos) {
@@ skipping 311 matching lines @@
       0xafff,   // 0x30 - 0x3f  1111111111110101
                 //              @ABCDEFGHIJKLMNO
       0xffff,   // 0x40 - 0x4f  1111111111111111
                 //              PQRSTUVWXYZ    _
       0x87ff,   // 0x50 - 0x5f  1111111111100001
                 //               abcdefghijklmno
       0xfffe,   // 0x60 - 0x6f  0111111111111111
                 //              pqrstuvwxyz   ~
       0x47ff];  // 0x70 - 0x7f  1111111111100010
 }