Specialize _toPow2String for bigint to make it linear instead of quadratic.

tests/corelib/corelib.status

 # 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.
 
 [ $compiler == dart2dart]
 bool_from_environment2_test: Skip
 bool_from_environment_default_value_test: Skip
 bool_from_environment_test: Skip
 from_environment_const_type_test: Skip
 from_environment_const_type_undefined_test: Skip
@@ skipping 187 matching lines @@
 from_environment_const_type_undefined_test/12: CompileTimeError
 from_environment_const_type_undefined_test/13: CompileTimeError
 from_environment_const_type_undefined_test/14: CompileTimeError
 from_environment_const_type_undefined_test/16: CompileTimeError
 
 [ $compiler == dart2analyzer ]
 int_parse_radix_test: fail
 list_insert_test: fail
 list_removeat_test: fail
 
-[ $arch == simarm || $arch == simarm64 ]
-big_integer_parsed_*: Pass, Slow
-
-[ $arch == simmips ]
-int_parse_radix_test: Skip # Timeout
-
 [ $arch == simmips || $arch == mips ]
-big_integer_parsed_*: Skip # Timeout. Issue 20879
-big_integer_huge_mul_vm_test: Skip # Timeout
-
-[ $arch == simarm && $checked ]
-num_parse_test: Skip  # Pass, Timeout
+big_integer_parsed_mul_div_vm_test: Pass, Slow  # Issue 20879.
 
 [ $compiler == dartanalyzer || $compiler == dart2analyzer ]
 error_stack_trace_test: StaticWarning, OK # Test generates errors on purpose.
 iterable_element_at_test: StaticWarning, OK # Test generates errors on purpose.
 num_clamp_test: StaticWarning, OK # Test generates errors on purpose.
 string_test: StaticWarning, OK # Test generates error on purpose.
 
 [ $runtime == ie10 ]
 num_parse_test: RuntimeError  # Issue 15316
 num_parse_test/01: RuntimeError  # Issue 15316
 
 [ $system == windows && $arch == x64 ]
 stopwatch_test: Skip  # Flaky test due to expected performance behaviour.
 
 [ $runtime != d8 ]
 # JSCRE does not pass all of the tests ported for irregexp.
 # Ignore all of the tests until irregexp is fully ported.
 regexp/*: Skip