Allow super only if class is extended (#200)

* jquery/esprima#2047

Author: jogibear9988

src/Esprima/JavascriptParser.cs

@@ -46,6 +46,7 @@ public Context()
             public bool IsModule;
             public bool AllowIn;
             public bool AllowStrictDirective;
+            public bool AllowSuper;
             public bool AllowYield;
             public bool IsAsync;
             public Token? FirstCoverInitializedNameError;
@@ -1581,7 +1582,7 @@ private Expression ParseLeftHandSideExpressionAllowCall()
                 expr = MatchKeyword("new") ? InheritCoverGrammar(parseNewExpression) : InheritCoverGrammar(parsePrimaryExpression);
             }
 
-            if (isSuper && !_context.InClassConstructor)
+            if (isSuper && (!_context.InClassConstructor || !_context.AllowSuper))
             {
                 TolerateError(Messages.UnexpectedSuper);
             }
@@ -4477,7 +4478,8 @@ private ClassDeclaration ParseClassDeclaration(bool identifierIsOptional = false
         {
             var node = CreateNode();
 
-            var previousStrict = _context.Strict;
+            var previousStrict = _context.Strict;
+            var previousAllowSuper = _context.AllowSuper;
             _context.Strict = true;
             ExpectKeyword("class");
 
@@ -4489,11 +4491,13 @@ private ClassDeclaration ParseClassDeclaration(bool identifierIsOptional = false
             if (MatchKeyword("extends"))
             {
                 NextToken();
-                superClass = IsolateCoverGrammar(ParseLeftHandSideExpressionAllowCall);
+                superClass = IsolateCoverGrammar(ParseLeftHandSideExpressionAllowCall);
+                _context.AllowSuper = true;
             }
 
             var classBody = ParseClassBody();
             _context.Strict = previousStrict;
+            _context.AllowSuper = previousAllowSuper;
 
             return Finalize(node, new ClassDeclaration(id, superClass, classBody));
         }