Use function param 'isclassdef', not function 'classdef' in conditional.
Accept "class C extends AssignmentExpression {}".

Examples:

class C extends func() {}
class C extends arr[0] {}
class C extends "str" {}
Make only class bodies strict by default, not the entire declaration.
Update tests to match new strictness scoping.
Mark 'interface' as an 'es5', 'strictOnly' keyword.
Remove unnecessary 'esnextReserved' meta tag.

This isn't needed if we adjust the code in `expression()` to only warn
about future reserved words if it really is a label.
`expression()` should only warn about future reserved words in labels.
Code cleanup: reduce diff with 'master'.
Remove explicit check for class names 'eval' or 'arguments'.
Add spec comment for strictness of 'ClassBody'.
Test "extends AssignmentExpression". Make line numbers easier to verify.

- Make the class declaration and class expression tests read more parallel.