Re-apply "Fix URL decoding in HTTP library"

runtime/bin/http_utils.dart

Index: runtime/bin/http_utils.dart
diff --git a/runtime/bin/http_utils.dart b/runtime/bin/http_utils.dart
index 3b120fd53ee2992bfd1bf3e54715b9b1379c9090..5414510bf38601c73da6c0e366a890841cb1932d 100644
--- a/runtime/bin/http_utils.dart
+++ b/runtime/bin/http_utils.dart
@@ -4,35 +4,46 @@
 
 class _HttpUtils {
   static String decodeUrlEncodedString(String urlEncoded) {
-    void invalidEscape() {
-      // TODO(sgjesse): Handle the error.
+    // First check the string for any encoding.
+    int index = 0;
+    bool encoded = false;
+    while (!encoded && index < urlEncoded.length) {
+      encoded = urlEncoded[index] == "+" || urlEncoded[index] == "%";
+      index++;
     }
+    if (!encoded) return urlEncoded;
+    index--;
 
-    StringBuffer result = new StringBuffer();
-    for (int ii = 0; urlEncoded.length > ii; ++ii) {
-      if ('+' == urlEncoded[ii]) {
-        result.add(' ');
-      } else if ('%' == urlEncoded[ii] &&
-                 urlEncoded.length - 2 > ii) {
-        try {
-          int charCode =
-            Math.parseInt('0x' + urlEncoded.substring(ii + 1, ii + 3));
-          if (charCode <= 0x7f) {
-            result.add(new String.fromCharCodes([charCode]));
-            ii += 2;
+    // Start decoding from the first encoded character.
+    List<int> bytes = new List<int>();
+    for (int i = 0; i < index; i++) bytes.add(urlEncoded.charCodeAt(i));
+    for (int i = index; i < urlEncoded.length; i++) {
+      if (urlEncoded[i] == "+") {
+        bytes.add(32);
+      } else if (urlEncoded[i] == "%") {
+        if (urlEncoded.length - i < 2) {
+          throw new HttpException("Invalid URL encoding");
+        }
+        int byte = 0;
+        for (int j = 0; j < 2; j++) {
+          var charCode = urlEncoded.charCodeAt(i + j + 1);
+          if (0x30 <= charCode && charCode <= 0x39) {
+            byte = byte * 16 + charCode - 0x30;
+          } else if (0x41 <= charCode && charCode <= 0x46) {
+            byte = byte * 16 + charCode - 0x37;
+          } else if (0x61 <= charCode && charCode <= 0x66) {
+            byte = byte * 16 + charCode - 0x57;
           } else {
-            invalidEscape();
-            return '';
+            throw new HttpException("Invalid URL encoding");
           }
-        } catch (BadNumberFormatException ignored) {
-          invalidEscape();
-          return '';
         }
+        bytes.add(byte);
+        i += 2;
       } else {
-        result.add(urlEncoded[ii]);
+        bytes.add(urlEncoded.charCodeAt(i));
       }
     }
-    return result.toString();
+    return decodeUtf8(bytes);
   }
 
   static Map<String, String> splitQueryString(String queryString) {