VM: extremely slow 'const' in some cases

[rakudrama]

A simple looking use of 'const' causes run time to nearly double.

Repro: https://chromiumcodereview.appspot.com/10060002/

Time before:
Started
Compiling minfrog on Dart VM took 11.8 seconds in checked mode + compile all
Compiling minfrog on Dart VM took 1.8 seconds in production mode
./frog/minfrog --out=frog/minfrog frog/minfrog.dart frog/tests/hello.dart
hello world
Bootstrapping minfrog took 3.4 seconds in production mode
Generated production minfrog is 507 kB

Time after:
Started
Compiling minfrog on Dart VM took 19.3 seconds in checked mode + compile all
Compiling minfrog on Dart VM took 3.5 seconds in production mode
./frog/minfrog --out=frog/minfrog frog/minfrog.dart frog/tests/hello.dart
hello world
Bootstrapping minfrog took 3.3 seconds in production mode
Generated production minfrog is 507 kB

[ghost]

The checked mode gets slower if a vari

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Set owner to @sgmitrovic.

[rakudrama]

Changing the test order to test thing === NEWLINE first recovers the performance.

        if (thing === NEWLINE) {
          sb.add('\n');
          pendingIndent = true;
        } else if (thing is String) {
          if (pendingIndent) {
            for (int i = 0; i < indentation; i++) {
              sb.add(INDENTATION);
            }
            pendingIndent = false;
          }
          sb.add(thing);
        } else

What this means is that the (x is String) test is very slow - O(100k) instructions when x is some random class.

[ghost]

Fixed in r6445

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Added Fixed label.

[rakudrama]

Added Verified label.