convert assert into a warning (#2178)

Author: jcollins-g

lib/src/model/getter_setter_combo.dart

@@ -10,6 +10,7 @@ import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/utils.dart';
+import 'package:dartdoc/src/warnings.dart';
 
 /// Mixin for top-level variables and fields (aka properties)
 mixin GetterSetterCombo on ModelElement {
@@ -47,14 +48,13 @@ mixin GetterSetterCombo on ModelElement {
 
   String linkifyConstantValue(String original) {
     if (constantInitializer is! InstanceCreationExpression) return original;
-    String constructorName = (constantInitializer as InstanceCreationExpression)
-        .constructorName
-        .toString();
-    Element staticElement =
-        (constantInitializer as InstanceCreationExpression).staticElement;
-    assert(staticElement != null,
-        '${original} should be able to be resolved but an analysis error prevents this');
-    if (staticElement == null) return original;
+    var creationExpression = constantInitializer as InstanceCreationExpression;
+    String constructorName = creationExpression.constructorName.toString();
+    Element staticElement = creationExpression.staticElement;
+    if (staticElement == null) {
+      warn(PackageWarning.missingConstantConstructor, message: constructorName);
+      return original;
+    }
     Constructor target = ModelElement.fromElement(staticElement, packageGraph);
     Class targetClass = target.enclosingElement;
     // TODO(jcollins-g): this logic really should be integrated into Constructor,

lib/src/model/package_graph.dart

@@ -418,6 +418,9 @@ class PackageGraph {
         warnablePrefix = 'for symbol';
         referredFromPrefix = 'conflicting with file already generated by';
         break;
+      case PackageWarning.missingConstantConstructor:
+        warningMessage = 'constant constructor missing: ${message}';
+        break;
     }
 
     List<String> messageParts = [warningMessage];

lib/src/warnings.dart

@@ -206,6 +206,19 @@ final Map<PackageWarning, PackageWarningDefinition> packageWarningDefinitions =
         "documentation comments to hide it."
       ],
       defaultWarningMode: PackageWarningMode.error),
+  PackageWarning.missingConstantConstructor: PackageWarningDefinition(
+      PackageWarning.missingConstantConstructor,
+      "missing-constant-constructor",
+      "Dartdoc can not show the value of a constant because its constructor could not be resolved.",
+      longHelp: [
+        "To resolve a constant into its literal value, Dartdoc relies on the",
+        "analyzer to resolve the constructor.  The analyzer didn't provide",
+        "the constructor for @@name@@, which is usually due to an error in the",
+        "code.  Use the analyzer to find missing imports.",
+      ],
+      // Defaults to ignore as this doesn't impact the docs severely but is
+      // useful for debugging package structure.
+      defaultWarningMode: PackageWarningMode.ignore),
 };
 
 /// Something that package warnings can be called on.  Optionally associated
@@ -244,6 +257,7 @@ enum PackageWarning {
   toolError,
   deprecated,
   unresolvedExport,
+  missingConstantConstructor,
 }
 
 /// Used to declare defaults for a particular package warning.