Don't copy text in _uriEncode if not necessary.

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 10aa827cb190c6e93a8889cad67312629145d5f5..944c8d38cb4c84624ec05278dca0a1e019512d74 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -2260,26 +2260,40 @@ class Uri {
                            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();