Introduce getter p on double in the VM to control fractional precision.

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;
 
@@ skipping 259 matching lines @@
       } else {
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
+
+  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;
 }