Simplify dart import syntax

[DartBot]

This issue was originally filed by [email protected]

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The dart import syntax can be simplified. I would have liked the go style import statements, which can cascade multiple imports and there is no need for the extra parenthesis over the name (the same can be done with source too). There is no need to imitate everything from java or other existing languages. This has hardly any learning curve.

For example

package main

import (
        "bufio"
        "flag"
        "fmt"
        "io"
        "math/rand"
        "os"
        "strings"
        "time"
)

looks much better than

library("swarmlib");

import('dart:coreimpl');

import('dart:json');

import('dart:html');

import('../../base/base.dart');

import('../../view/view.dart');

import('../../observable/observable.dart');

import('../../touch/touch.dart');

import('../../util/utilslib.dart');

source('App.dart');

source('BiIterator.dart');

source('ConfigHintDialog.dart');

source('HelpDialog.dart');

source('SwarmState.dart');

source('SwarmViews.dart');

source('SwarmApp.dart');

source('DataSource.dart');

source('Decoder.dart');

source('UIState.dart');

source('Views.dart');

source('CSS.dart');

[dgrove]

Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

Added Accepted label.

[gbracha]

If it's going to change, we need to decide before M1.

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Added this to the M1 milestone.

[gbracha]

Issue #3858 has been merged into this issue.

[gbracha]

See spec draft 0.11

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Added Done label.

[gbracha]

Analyzer issue #4048.
VM issue #4049.
dart2js issue #4050.

See spec draft version 0.11 and above, section 12.