Panic on the input foo(a=)

[connorskees]

For the following input dart-sass will raise an exception,

a {
    color: foo(a=);
}

Unexpected exception:
NoSuchMethodError: The method 'accept' was called on null.
Receiver: null
Tried calling: accept<Value>(Instance of '_EvaluateVisitor')


dart:core                                                         Object.noSuchMethod
package:sass/src/visitor/evaluate.dart 1909:34                    _EvaluateVisitor.visitBinaryOperationExpression.<fn>
package:sass/src/visitor/evaluate.dart 2945:22                    _EvaluateVisitor._addExceptionSpan
package:sass/src/visitor/evaluate.dart 1905:12                    _EvaluateVisitor.visitBinaryOperationExpression
package:sass/src/ast/sass/expression/binary_operation.dart 52:15  BinaryOperationExpression.accept
package:sass/src/visitor/evaluate.dart 2754:29                    _EvaluateVisitor._evaluateToCss
package:sass/src/visitor/evaluate.dart 2206:22                    _EvaluateVisitor._runFunctionCallable
package:sass/src/visitor/evaluate.dart 2077:21                    _EvaluateVisitor.visitFunctionExpression.<fn>
package:sass/src/visitor/evaluate.dart 2963:22                    _EvaluateVisitor._addErrorSpan
package:sass/src/visitor/evaluate.dart 2076:18                    _EvaluateVisitor.visitFunctionExpression
package:sass/src/ast/sass/expression/function.dart 38:15          FunctionExpression.accept
package:sass/src/visitor/evaluate.dart 1037:31                    _EvaluateVisitor.visitDeclaration
package:sass/src/ast/sass/statement/declaration.dart 26:55        Declaration.accept
package:sass/src/visitor/evaluate.dart 1734:17                    _EvaluateVisitor.visitStyleRule.<fn>.<fn>
package:sass/src/visitor/evaluate.dart 2837:26                    _EvaluateVisitor._withStyleRule
package:sass/src/visitor/evaluate.dart 1732:7                     _EvaluateVisitor.visitStyleRule.<fn>
package:sass/src/environment.dart 754:24                          Environment.scope
package:sass/src/visitor/evaluate.dart 2801:31                    _EvaluateVisitor._withParent
package:sass/src/visitor/evaluate.dart 1731:5                     _EvaluateVisitor.visitStyleRule
package:sass/src/ast/sass/statement/style_rule.dart 26:55         StyleRule.accept
package:sass/src/visitor/evaluate.dart 862:13                     _EvaluateVisitor.visitStylesheet
package:sass/src/visitor/evaluate.dart 678:7                      _EvaluateVisitor._execute.<fn>
package:sass/src/visitor/evaluate.dart 2700:26                    _EvaluateVisitor._withEnvironment
package:sass/src/visitor/evaluate.dart 648:5                      _EvaluateVisitor._execute
package:sass/src/visitor/evaluate.dart 486:20                     _EvaluateVisitor.run.<fn>
package:sass/src/warn.dart 32:20                                  withWarnCallback.<fn>
dart:async                                                        runZoned
package:sass/src/warn.dart 31:10                                  withWarnCallback
package:sass/src/visitor/evaluate.dart 502:12                     _EvaluateVisitor._withWarnCallback
package:sass/src/visitor/evaluate.dart 473:12                     _EvaluateVisitor.run
package:sass/src/visitor/evaluate.dart 95:10                      evaluate
package:sass/src/compile.dart 134:24                              _compileStylesheet
package:sass/src/compile.dart 64:10                               compile
package:sass/src/executable/compile_stylesheet.dart 91:13         compileStylesheet