Properly indicate the error location in the REPL

Previously, errors in variable assignment lines would be reported
incorrectly.

lib/src/executable/repl.dart

@@ -12,6 +12,7 @@ import '../ast/sass.dart';
 import '../exception.dart';
 import '../executable/options.dart';
 import '../logger/tracking.dart';
+import '../parse/parser.dart';
 import '../value.dart' as internal;
 import '../visitor/evaluate.dart';
 
@@ -23,14 +24,12 @@ Future<void> repl(ExecutableOptions options) async {
     if (line.trim().isEmpty) continue;
     var logger = TrackingLogger(options.logger);
     try {
-      Expression expression;
       VariableDeclaration declaration;
-      try {
+      Expression expression;
+      if (Parser.isVariableDeclarationLike(line)) {
         declaration = VariableDeclaration.parse(line, logger: logger);
         expression = declaration.expression;
-      } on SassFormatException {
-        // TODO(nweiz): If [line] looks like a variable assignment, rethrow the
-        // original exception.
+      } else {
         expression = Expression.parse(line, logger: logger);
       }
 

lib/src/parse/parser.dart

@@ -41,6 +41,12 @@ class Parser {
     }
   }
 
+  /// Returns whether [text] starts like a variable declaration.
+  ///
+  /// Ignores everything after the `:`.
+  static bool isVariableDeclarationLike(String text, {Logger logger}) =>
+      Parser(text, logger: logger)._isVariableDeclarationLike();
+
   @protected
   Parser(String contents, {url, Logger logger})
       : scanner = SpanScanner(contents, sourceUrl: url),
@@ -54,6 +60,14 @@ class Parser {
     });
   }
 
+  bool _isVariableDeclarationLike() {
+    if (!scanner.scanChar($dollar)) return false;
+    if (!lookingAtIdentifier()) return false;
+    identifier();
+    whitespace();
+    return scanner.scanChar($colon);
+  }
+
   // ## Tokens
 
   /// Consumes whitespace, including any comments.

test/cli/shared/repl.dart

@@ -89,6 +89,19 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
       await sass.kill();
     });
 
+    test("a parse error in a variable declaration", () async {
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln("\$foo: 1 + 2;");
+      await expectLater(
+          sass.stdout,
+          emitsInOrder([
+            ">> \$foo: 1 + 2;",
+            "              ^",
+            "Error: expected no more input."
+          ]));
+      await sass.kill();
+    });
+
     test("a parse error after the end of the input", () async {
       var sass = await runSass(["--interactive"]);
       sass.stdin.writeln("foo(");