Implement string interpolation up to SSA.

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 851eca544f7f986013e5ace317d5cffe6cd756e6..d480547706793768f8c012ba030c28e41f7b5c89 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -954,4 +954,13 @@ class SsaBuilder implements Visitor {
     joinBlock.addPhi(phi);
     stack.add(phi);
   }
+
+  visitStringInterpolation(StringInterpolation node) {
+    compiler.unimplemented('SsaBuilder.visitStringInterpolation', node: node);
+  }
+
+  visitStringInterpolationPart(StringInterpolationPart node) {
+    compiler.unimplemented('SsaBuilder.visitStringInterpolationPart',
+                           node: node);
+  }
 }