Fraction class prototype and test (not to be committed).

runtime/vm/scanner.cc

Index: runtime/vm/scanner.cc
diff --git a/runtime/vm/scanner.cc b/runtime/vm/scanner.cc
index f7ae4ac59525fca9ca450cce9d6fa356af548dbb..b88770842084d71c2a3f0d0a33784e4f55a66e45 100644
--- a/runtime/vm/scanner.cc
+++ b/runtime/vm/scanner.cc
@@ -344,9 +344,10 @@ void Scanner::ScanIdentChars(bool allow_dollar) {
 }
 
 
-// Parse integer or double number literal of format:
-// NUMBER = INTEGER | DOUBLE
+// Parse integer, rational, or double number literal of format:
+// NUMBER = INTEGER | RATIONAL | DOUBLE
 // INTEGER = D+ | (("0x" | "0X") H+)
+// RATIONAL = ((D+ ["." D*]) | ("." D+)) ("r" | "R")
 // DOUBLE = ((D+ ["." D*]) | ("." D+)) [ EXPONENT ]
 // EXPONENT = ("e" | "E") ["+" | "-"] D+
 void Scanner::ScanNumber(bool dec_point_seen) {
@@ -388,6 +389,8 @@ void Scanner::ScanNumber(bool dec_point_seen) {
       while (IsDecimalDigit(c0_)) {
         ReadChar();
       }
+    } else if ((c0_ == 'r') || (c0_ == 'R')) {
+      Recognize(Token::kRATIONAL);
     }
   }
   if (current_token_.kind != Token::kILLEGAL) {