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

runtime/lib/double.dart

Index: runtime/lib/double.dart
diff --git a/runtime/lib/double.dart b/runtime/lib/double.dart
index ade4b9317bcd75c3c5150c78372a803f8630516d..34d780341d79f5626343c093d839ea07332d3f95 100644
--- a/runtime/lib/double.dart
+++ b/runtime/lib/double.dart
@@ -277,4 +277,11 @@ class _Double implements double {
       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;
 }