Simplify double to bigint conversion.

runtime/lib/integers.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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
 class _IntegerImplementation extends _Num {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
 
@@ skipping 182 matching lines @@
     }
     if (lowerLimit.isNaN) return lowerLimit;
     // Note that we don't need to care for -0.0 for the lower limit.
     if (this < lowerLimit) return lowerLimit;
     if (this.compareTo(upperLimit) > 0) return upperLimit;
     return this;
   }
 
   int toInt() { return this; }
   double toDouble() { return new _Double.fromInteger(this); }
-  _Bigint _toBigint() { return new _Bigint()._setInt(this); }
+  _Bigint _toBigint() { return new _Bigint._fromInt(this); }
   num _toBigintOrDouble() { return _toBigint(); }
 
   String toStringAsFixed(int fractionDigits) {
     return this.toDouble().toStringAsFixed(fractionDigits);
   }
   String toStringAsExponential([int fractionDigits]) {
     return this.toDouble().toStringAsExponential(fractionDigits);
   }
   String toStringAsPrecision(int precision) {
     return this.toDouble().toStringAsPrecision(precision);
@@ skipping 289 matching lines @@
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }