do not try to classify missing nodes

src/services/services.ts

@@ -6300,6 +6300,10 @@ namespace ts {
             }
 
             function classifyToken(token: Node): void {
+                if (nodeIsMissing(token)) {
+                    return;
+                }
+
                 let tokenStart = classifyLeadingTriviaAndGetTokenStart(token);
 
                 let tokenWidth = token.end - tokenStart;

tests/cases/fourslash/syntacticClassificationWithErrors.ts

@@ -0,0 +1,14 @@
+/// <reference path='fourslash.ts'/>
+
+////class A {
+////    a:
+////}
+////c =
+
+let c = classification
+verify.syntacticClassificationsAre(
+    c.keyword("class"), c.className("A"), c.punctuation("{"),
+    c.text("a"), c.punctuation(":"),
+    c.punctuation("}"),
+    c.text("c"), c.operator("=")
+    );