Avoid unnecessary copying when parsing doubles.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index bc9b4d8be6d85a2ebbbc96016c48e8dfe06a77b2..c5ed491013dba18c11c69ad2f9fcc0dbff78e44e 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -128,7 +128,6 @@ class _StringBase {
     return true;
   }
 
-
   int compareTo(String other) {
     int thisLength = this.length;
     int otherLength = other.length;
@@ -294,7 +293,7 @@ class _StringBase {
         (codePoint == 0xFEFF);
   }
 
-  String trim() {
+  int _firstNonWhitespace() {
     final len = this.length;
     int first = 0;
     for (; first < len; first++) {
@@ -302,22 +301,33 @@ class _StringBase {
         break;
       }
     }
-    if (len == first) {
-      // String contains only whitespaces.
-      return "";
-    }
-    int last = len - 1;
-    for (; last >= first; last--) {
+    return first;
+  }
+
+  int _lastNonWhitespace() {
+    int last = this.length - 1;
+    for (; last >= 0; last--) {

[Ivan Posva, 2014/07/03 19:44:14]

I know this was used in trim before, but I think it predates the switch away
from 4-byte string encodings. Is it legal to scan backwards for white space with
the current use of surrogate pairs?

[Lasse Reichstein Nielsen, 2014/07/04 06:46:01]

Yes. All Unicode white-space are in the basic multilingual plane, so no
surrogate is part of a white-space character.

       if (!_isWhitespace(this.codeUnitAt(last))) {
         break;
       }
     }
-    if ((first == 0) && (last == (len - 1))) {
+    return last;
+  }
+
+  String trim() {
+    final len = this.length;
+    int first = _firstNonWhitespace();
+    if (len == first) {
+      // String contains only whitespaces.
+      return "";
+    }
+    int last = _lastNonWhitespace() + 1;
+    if ((first == 0) && (last == len)) {
       // Returns this string since it does not have leading or trailing
       // whitespaces.
       return this;
     }
-    return _substringUnchecked(first, last + 1);
+    return _substringUnchecked(first, last);
   }
 
   String trimLeft() {