Revert "Revert "Recognize and optimize a.runtimeType == b.runtimeType pattern.""

runtime/lib/double.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _Double implements double {
   factory _Double.fromInteger(int value)
       native "Double_doubleFromInteger";
 
-  Type get runtimeType => double;
-
   // TODO: Make a stared static method for hashCode and _identityHashCode
   //       when semantics are corrected as described in:
   //       https://github.com/dart-lang/sdk/issues/2884
   int get hashCode => (isNaN || isInfinite) ?  0 : toInt();
   int get _identityHashCode => (isNaN || isInfinite) ?  0 : toInt();
 
   double operator +(num other) {
     return _add(other.toDouble());
   }
   double _add(double other) native "Double_add";
@@ skipping 257 matching lines @@
     }
   }
 
   static const int _FRACTIONAL_BITS = // Bits to keep after the decimal point.
       const int.fromEnvironment("doubleFractionalBits", defaultValue: 20);
   static const double _BIAS = 1.5 * (1 << (52 - _FRACTIONAL_BITS));
 
   // Returns this with only _FRACTIONAL_BITS bits after the decimal point.
   double get p => this + _BIAS - _BIAS;
 }