fix(@ngtools/webpack): diagnose typescript warnings as warnings

Previously, we were reporting all diagnostics as errors. Now only Typescript errors
will be reported as real errors to webpack. Messages and warnings will be warnings.

Fixes #5623

packages/@ngtools/webpack/src/plugin.ts

@@ -361,8 +361,8 @@ export class AotPlugin implements Tapable {
 
       const original = consumer.originalPositionFor({ line, column: character });
       return {
-        line: original.line,
-        character: original.column,
+        line: typeof original.line == 'number' ? original.line : line,
+        character: typeof original.column == 'number' ? original.column : character,
         fileName: original.source || fileName
       };
     } catch (e) {
@@ -390,19 +390,25 @@ export class AotPlugin implements Tapable {
       );
 
     if (diagnostics.length > 0) {
-      const message = diagnostics
-        .map(diagnostic => {
-          const position = diagnostic.file.getLineAndCharacterOfPosition(diagnostic.start);
-
-          const sourceText = diagnostic.file.getFullText();
-          let {line, character, fileName} = this._translateSourceMap(sourceText,
-            diagnostic.file.fileName, position);
-
-          const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');
-          return `${fileName} (${line + 1},${character + 1}): ${message}`;
-        })
-        .join('\n');
-      this._compilation.errors.push(message);
+      diagnostics.forEach(diagnostic => {
+        const position = diagnostic.file.getLineAndCharacterOfPosition(diagnostic.start);
+
+        const sourceText = diagnostic.file.getFullText();
+        let {line, character, fileName} = this._translateSourceMap(sourceText,
+          diagnostic.file.fileName, position);
+
+        const messageText = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');
+        const message = `${fileName} (${line + 1},${character + 1}): ${messageText}`;
+
+        switch (diagnostic.category) {
+          case ts.DiagnosticCategory.Error:
+            this._compilation.errors.push(message);
+            break;
+
+          default:
+            this._compilation.warnings.push(message);
+        }
+      });
     }
   }