Short-circuit Uri._uriEncode for components that don't need encoding.

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..3c458358d690fe9bf723fd914764e049efde4d85 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -2260,6 +2260,8 @@ class Uri {
                            String text,
                            {Encoding encoding: UTF8,
                             bool spaceToPlus: false}) {
+    if (_noUriEncodingNeeded(canonicalTable, text, encoding)) return text;
+
     byteToHex(byte, buffer) {
       const String hex = '0123456789ABCDEF';
       buffer.writeCharCode(hex.codeUnitAt(byte >> 4));
@@ -2285,6 +2287,17 @@ class Uri {
     return result.toString();
   }
 
+  static bool _noUriEncodingNeeded(
+      List<int> canonicalTable, String text, Encoding encoding) {
+    if (encoding != UTF8) return false;
+    for (int i = 0; i < text.length; i++) {
+      int code = text.codeUnitAt(i);
+      if (code >= 128) return false;
+      if ((canonicalTable[code >> 4] & (1 << (code & 0x0f))) == 0) return false;
+    }
+    return true;
+  }
+
   /**
    * Convert a byte (2 character hex sequence) in string [s] starting
    * at position [pos] to its ordinal value