Improve perfromance of integer parsing for Smi-s.

runtime/lib/integers.cc

Index: runtime/lib/integers.cc
===================================================================
--- runtime/lib/integers.cc	(revision 18983)
+++ runtime/lib/integers.cc	(working copy)
@@ -183,6 +183,26 @@
 
 DEFINE_NATIVE_ENTRY(Integer_parse, 1) {
   GET_NON_NULL_NATIVE_ARGUMENT(String, value, arguments->NativeArgAt(0));
+  if (value.IsOneByteString()) {
+    // Quick conversion for unpadded integers in strings.
+    const intptr_t len = value.Length();
+    if (len > 0) {
+      const char* cstr = value.ToCString();
+      ASSERT(cstr != NULL);
+      // Dart differences from strtol:
+      // a) '+5' is not a valid integer (leading plus).
+      if (cstr[0] != '+') {
+        char* p_end = NULL;
+        const int64_t int_value = strtol(cstr, &p_end, 10);
+        if (p_end == (cstr + len)) {
+          if ((Smi::kMinValue <= int_value) && (int_value <= Smi::kMaxValue)) {
+            return Smi::New(int_value);
+          }
+        }
+      }
+    }
+  }
+
   Scanner scanner(value, Symbols::Empty());
   const Scanner::GrowableTokenStream& tokens = scanner.GetStream();
   String* int_string;