JS: Extractor: More robust ES2015 checking

Created shared AbstractDetector to not duplicate all the tedious logic
;)

I took inspiration from the tests in  `javascript/extractor/tests/esnext/input/dynamic-import.js`

javascript/extractor/src/com/semmle/js/extractor/ES2015Detector.java

@@ -0,0 +1,34 @@
+package com.semmle.js.extractor;
+
+import com.semmle.js.ast.DynamicImport;
+import com.semmle.js.ast.ExportDeclaration;
+import com.semmle.js.ast.Expression;
+import com.semmle.js.ast.ImportDeclaration;
+import com.semmle.js.ast.Node;
+import com.semmle.js.ast.Statement;
+
+/** A utility class for detecting Node.js code. */
+public class ES2015Detector extends AbstractDetector {
+  /**
+   * Is {@code ast} a program that uses ES2015 import/export code?
+   */
+  public static boolean looksLikeES2015(Node ast) {
+    return new ES2015Detector().programDetection(ast);
+  }
+
+  @Override
+  protected boolean visitStatement(Statement stmt) {
+    if (stmt instanceof ImportDeclaration || stmt instanceof ExportDeclaration) {
+      return true;
+    }
+    return super.visitStatement(stmt);
+  }
+
+  @Override
+  protected boolean visitExpression(Expression e) {
+    if (e instanceof DynamicImport) {
+      return true;
+    }
+    return super.visitExpression(e);
+  }
+}

javascript/extractor/src/com/semmle/js/extractor/JSExtractor.java

@@ -58,6 +58,26 @@ public class JSExtractor {
 
     JSParser.Result parserRes =
         JSParser.parse(config, sourceType, source, textualExtractor.getMetrics());
+
+    // Check if we guessed wrong with the regex in `establishSourceType`, (which could
+    // happen due to a block-comment line starting with '  import').
+    if (config.getSourceType() == SourceType.AUTO && sourceType != SourceType.SCRIPT) {
+      boolean wrongGuess = false;
+
+      if (sourceType == SourceType.MODULE) {
+        // check that we did see an import/export declaration
+        wrongGuess = ES2015Detector.looksLikeES2015(parserRes.getAST()) == false;
+      } else if (sourceType == SourceType.CLOSURE_MODULE ) {
+        // TODO
+      }
+
+      if (wrongGuess) {
+        sourceType = SourceType.SCRIPT;
+        parserRes =
+          JSParser.parse(config, sourceType, source, textualExtractor.getMetrics());
+      }
+    }
+
     return extract(textualExtractor, source, toplevelKind, scopeManager, sourceType, parserRes);
   }