Fix ES2018 import()

src/Esprima/Ast/Import.cs

@@ -4,10 +4,17 @@ namespace Esprima.Ast
 {
     public sealed class Import : Expression
     {
+        public readonly Expression Source;
+
         public Import() : base(Nodes.Import)
         {
         }
 
+        public Import(Expression source) : base(Nodes.Import)
+        {
+            Source = source;
+        }
+
         public override NodeCollection ChildNodes => NodeCollection.Empty;
 
         protected internal override void Accept(AstVisitor visitor)

src/Esprima/JavascriptParser.cs

@@ -1510,8 +1510,24 @@ private bool MatchImportCall()
         private Import ParseImportCall()
         {
             var node = CreateNode();
-            ExpectKeyword("import");
-            return Finalize(node, new Import());
+            ExpectKeyword("import");
+            Expect("(");
+
+            var source = this.parseAssignmentExpression();
+            if (!this.Match(")") && this._config.Tolerant)
+            {
+                this.TolerateUnexpectedToken(this.NextToken());
+            }
+            else
+            {
+                this.Expect(")");
+                if (this.Match(";"))
+                {
+                    this.NextToken();
+                }
+            }
+
+            return Finalize(node, new Import(source));
         }
 
         private bool MatchImportMeta()

src/Esprima/Utils/AstVisitor.cs

@@ -330,7 +330,8 @@ protected internal virtual void VisitExportSpecifier(ExportSpecifier exportSpeci
         }
 
         protected internal virtual void VisitImport(Import import)
-        {
+        {
+            Visit(import.Source);
         }
 
         protected internal virtual void VisitImportDeclaration(ImportDeclaration importDeclaration)