Nicer failures when an annotation is not resolvable

CHANGELOG.md

@@ -5,6 +5,10 @@
   result we've added `#annotationsOfExact|firstAnnotationOfExact` which has the
   old behavior for precise checks.
 
+* `TypeChecker#annotations...`-methods now throw a `StateError` if one or more
+  annotations on an element are not resolvable. This is usually a sign of a
+  mispelling, missing import, or missing dependency.
+
 ## 0.5.10+1
 
 * Update minimum `analyzer` package to `0.29.10`.

lib/src/type_checker.dart

@@ -58,16 +58,27 @@ abstract class TypeChecker {
     return results.isEmpty ? null : results.first;
   }
 
+  DartObject _checkedConstantValue(ElementAnnotation annotation) {
+    final result = annotation.computeConstantValue();
+    if (result == null) {
+      throw new StateError(
+          'Could not resolve $annotation. An import or dependency may be '
+          'missing or invalid.');
+    }
+    return result;
+  }
+
   /// Returns annotating constants on [element] assignable to this type.
   Iterable<DartObject> annotationsOf(Element element) => element.metadata
-      .map((a) => a.computeConstantValue())
+      .map(_checkedConstantValue)
       .where((a) => isAssignableFromType(a.type));
 
   /// Returns annotating constants on [element] of exactly this type.
   Iterable<DartObject> annotationsOfExact(Element element) => element.metadata
-      .map((a) => a.computeConstantValue())
-      .where((a) => isExactlyType(a.type));
+      .map(_checkedConstantValue)
+      .where((a) => a?.type != null && isAssignableFromType(a.type));
 
+  /// Returns `true` if the type of [element] can be assigned to this type.
   /// Returns `true` if the type of [element] can be assigned to this type.
   bool isAssignableFrom(Element element) =>
       isExactly(element) || _getAllSupertypes(element).any(isExactlyType);

test/type_checker_test.dart

@@ -179,4 +179,19 @@ void main() {
         checkGeneratorForAnnotation: () => const TypeChecker.fromUrl(
             'package:source_gen/src/generator_for_annotation.dart#GeneratorForAnnotation'));
   });
+
+  test('should gracefully when something is not resolvable', () async {
+    final resolver = await resolveSource(r'''
+      library _test;
+
+      @depracated // Intentionally mispelled.
+      class X {}
+    ''');
+    final lib = resolver.getLibraryByName('_test');
+    final classX = lib.getType('X');
+    final $deprecated = const TypeChecker.fromRuntime(Deprecated);
+
+    expect(() => $deprecated.annotationsOf(classX), throwsStateError,
+        reason: 'deprecated was spelled wrong; no annotation can be resolved');
+  });
 }