dart2js: don't emit big integers as integer, but instead use their exponential representation.

pkg/compiler/lib/src/js_backend/constant_emitter.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.
 
 part of js_backend;
 
 class ConstantEmitter {
   ConstantReferenceEmitter _referenceEmitter;
   ConstantLiteralEmitter _literalEmitter;
 
@@ skipping 184 matching lines @@
     return null;
   }
 
   @override
   jsAst.Expression visitNull(NullConstantValue constant, [_]) {
     return new jsAst.LiteralNull();
   }
 
   @override
   jsAst.Expression visitInt(IntConstantValue constant, [_]) {
-    return new jsAst.LiteralNumber('${constant.primitiveValue}');
+    int primitiveValue = constant.primitiveValue;
+    // Since we are in JavaScript we can shorten long integers to their
+    // shorter exponential representation.
+    // For example: "1e+4" is shorter than "10000".

[sra1, 2015/02/20 17:02:54]

The + is not necessary, making "1e3" shorter than "1000".
The period is never necessary for integer values:

1.2e+10 ->  12e9

I might be a bit nervous about hacking the string output outside the
exact-integer (2^53) range, but the parser should be just counting digits to
adjust the exponent before dividing.

Make sure all this works when running dart2js self-compiled, when the 'int' is
represented as an integer valued double.  This basically means you can't rely on
the int toString() returning only digits.

[floitsch, 2015/02/20 20:23:20]

https://codereview.chromium.org/938323003

+    //
+    // Note that this shortening apparently loses precision for big numbers
+    // (like 1234567890123456789012345 which becomes 1.2345678901234568e+24).
+    // However, since JavaScript engines implicitly convert to double, these

[sra1, 2015/02/20 17:02:54]

nit. They don't 'implicitly convert to' so much as 'represent as'.

[floitsch, 2015/02/20 20:23:20]

done in https://codereview.chromium.org/938323003

+    // digits are lost anyway.
+    if (primitiveValue.abs() >= 10000) {
+      String exponential = primitiveValue.toStringAsExponential();
+      String decimal = primitiveValue.toString();
+      return new jsAst.LiteralNumber(
+          (exponential.length < decimal.length) ? exponential : decimal);
+    }
+    return new jsAst.LiteralNumber('$primitiveValue');
   }
 
   @override
   jsAst.Expression visitDouble(DoubleConstantValue constant, [_]) {
     double value = constant.primitiveValue;
     if (value.isNaN) {
       return js("0/0");
     } else if (value == double.INFINITY) {
       return js("1/0");
     } else if (value == -double.INFINITY) {
@@ skipping 182 matching lines @@
                             [value, argumentList]);
     }
     return value;
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantEmitter.reference(constant.referenced);
   }
 }