Introduce element categories and move resolver towards being more compositional.

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index b337d71a63d47f300dd5a82b6ef7cf19f27b02ad..246bfb264dccb04e0922e2f1e6a82b64259646b7 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -1781,6 +1781,11 @@ class SsaBuilder implements Visitor {
   visitSuperSend(Send node) {
     Selector selector = elements.getSelector(node);
     Element element = elements[node];
+    if (element.kind.category != ElementCategory.FUNCTION) {
+      // Example:
+      // co19/Language/10_Expressions/14_Method_Invocation/3_Super_Invocation_A03_t04
+      compiler.unimplemented('super-send to non-function', node: node);
+    }
     HStatic target = new HStatic(element);
     HInstruction context = localsHandler.readThis();
     add(target);
@@ -2073,7 +2078,8 @@ class SsaBuilder implements Visitor {
   }
 
   visitStringInterpolation(StringInterpolation node) {
-    Operator op = new Operator.synthetic("+");
+    int offset = node.getBeginToken().charOffset;
+    Operator op = new Operator(new StringToken(PLUS_INFO, "+", offset));
     HInstruction target = new HStatic(interceptors.getOperatorInterceptor(op));
     add(target);
     visit(node.string);