Put new global var semantics behind a flag until WebKit tests are cleaned up.

test/mjsunit/regress/regress-1170.js

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+// Flags: --es52_globals
+
 var setter_value = 0;
 
 this.__defineSetter__("a", function(v) { setter_value = v; });
 eval("var a = 1");
 assertEquals(1, setter_value);
 assertFalse("value" in Object.getOwnPropertyDescriptor(this, "a"));
 
 eval("with({}) { eval('var a = 2') }");
 assertEquals(2, setter_value);
 assertFalse("value" in Object.getOwnPropertyDescriptor(this, "a"));
@@ skipping 49 matching lines @@
   eval("const dd = 23");
 } catch(e) {
   assertUnreachable();
 }
 
 try {
   eval("with({}) { eval('const dd = 23') }");
 } catch(e) {
   assertInstanceof(e, TypeError);
 }