The following nodes were instances of AST_BlockStatement: AST_Scope,
AST_SwitchBlock, AST_SwitchBranch. Also, AST_Try, AST_Catch, AST_Finally
were having a body instanceof AST_BlockStatement.
Overloading the meaning of AST_BlockStatement this way turned out to be a
mess; we now have an AST_Block class that is the base class for things
having a block of statements (might or might not be bracketed). The
`this.body` of AST_Scope, AST_Try, AST_Catch, AST_Finally is now an array of
statements (as they inherit from AST_Block).
Avoiding calling superclass's _walk function in walkers (turns out we walked
a node multiple times).
optimize away while(false), and transform while(true) ==> for(;;).
UNSAFE:
some expressions are optimized away when we're in boolean context and can
determine that the value will always be true or false. For example:
x() || true ==> always `true` in boolean context
x() && false ==> always `false` in boolean context
It's not technically correct to drop these expressions since we drop the
function call too (that might have side effects); on the other hand, I can't
see any legitimate use for such expressions and they might simply indicate a
bug (we do warn about it).
